The sound that we hear in a room is a complex combination of the direct sound and the sound indirectly scattered from the room's contents and boundary surfaces. The indirect reflections can be manipulated by reflection, diffusion and absorption. Various porous materials have been used to provide sound absorption, such as fiberglass batting, various woven and non-woven cloths, rugs, etc. One of the most widely used materials for the purpose of sound absorption has been plastic foams. Plastic foams manufactured from various resins such as polyester urethane and polyether urethane have existed for almost fifty years and they have been used widely for sound absorption for at least the latter half of that period. These urethane foams do not meet the Class A Life Safety Code 6-5.3.2 for an interior wall and ceiling finish. Typically urethane foams are a Class C material with a flame spread between 76-200 and a smoke developed of 0-450.
Class A includes any material classified at 25 or less on the flame spread test scale and 450 or less on the smoke test scale described in National Fire Protection Association standard 255 or ASTM E-84. Newer foams such as melamine and polyimide have begun to find their way into the acoustical absorption market and do meet the requirements of Class A. However, melamine and polyimide are much more expensive than polyurethane. Polyurethanes are relatively low cost materials, costing approximately $0.70/lb. Melamine, although not much more in cost than polyurethane, usually under $1.00/lb., has proven difficult and costly to obtain in a foamed state. As a foamed product, melamine is almost twice the price of a comparable polyurethane product of similar design and configuration. Polyimide at $18.00/lb. and other newer foams have the severely limiting factor of high cost. This has left the polyurethane and melamine foams to fill most acoustical absorption processing needs.
Traditional foam absorption products consist of a flat rear surface which is glued to a reflecting room boundary and a front surface which usually has some unique design formed by a computer numerically controlled (CNC) cutter or a convoluting apparatus. The active surface is designed to both increase the total surface area for greater potential absorption as well as to create some aesthetic value. This essential design, with a large flat surface directly adhered to the reflective room boundary has been one design aspect which has been included in all of the acoustical tiles on the market. Applicant has found that for optimum material utilization and sound absorption, a porous sound absorbing material should not be placed directly on a reflective surface.
A porous material absorbs sound by converting sound energy into heat by friction of the vibrating air particles within the fine pores of the material. For this process to be effective, there must be freedom for the air particles to move. The higher the particle velocity, the better the sound absorbing capability. As the particle velocity is decreased, energy conversion is less efficient and less sound energy is absorbed. At a hard wall surface, the particle velocity is zero, hence there is very little absorption. Thus, any sound absorptive material placed against a hard wall is virtually useless because there can be no air motion within and behind the material to dissipate the sound energy.
Nevertheless, it is common practice to mount sound absorptive layers directly against a wall because it is very convenient to do so. Applicant has discovered that, in such cases, only a fraction of the outer layer is effective in absorbing sound below 1000 Hz. The rest of the material is simply acting as a convenient support. Since the price of melamine, polyimide and other future fire-safe foams is significant, it is important to fully utilize as much of the volume of the foam material for absorption as is possible.
It is with these problems encountered with foam absorbing materials as used in the prior art that the present invention was developed.